Also, I assume this may have been looked at before but maybe ranking in general could be based on the word count or character count of posts as opposed to the quantity of them? Maybe too resource intensive?
I see alot of spammers making long wordy posts with little content, If your system is adopted, those users will rank quickly although they are spamming.
Also, the forum had to change it's activity update system because it was taking up too many resources and a few days ago theymos had to disable the view members page for the same reason. Your plan will need too many resources.